The present invention relates generally to the hydraulic pressure generation in a vehicle brake system. The invention is directed in particular to a pressure generator with a multiple-piston pump.
Modern hydraulic or electrohydraulic vehicle brake systems require reliable pressure generators in order to be able to implement safety-related functions such as preventing locking of the brake or preventing spinning wheels. For this purpose, driver assistance systems, such as anti-lock control (ABS), anti-slip control (ASR) or driving dynamics control and adaptive speed control (ACC), cooperate with a pressure generator of the vehicle brake system.
Known brake systems of this type work with a hydraulic fluid as the brake fluid, which is partly stored in a storage container provided therefor. To generate brake pressure, the brake fluid is put under pressure in a brake circuit and in this way acts to activate one or more wheel brakes. In the case of a braking procedure initiated by a driver (referred to hereinafter as driver braking), a master brake cylinder actuated by the driver pressing a foot pedal serves this purpose. In contrast to this, in the case of braking which is not requested by the driver but by a driver assistance system (referred to hereinafter as system braking), the brake pressure required is generated by a pressure generator. A third possible operating state of a vehicle brake system of this type is mixed operation, i.e. braking which is initiated both by the driver and by the system (referred to hereinafter as mixed braking).
For a better understanding, a vehicle brake system from the prior art is described below. According to FIG. 1, this system has two brake circuits A and B which are of substantially identical construction. Therefore, only the first brake circuit A which supplies two wheel brakes 50 and 60 is described in more detail.
In the case of driver braking, a master cylinder 12 delivers a brake fluid from a storage container 14 to the two brake circuits A and B. Between the master cylinder 12 and the wheel brakes 50 and 60 there is a hydraulic connection which is controlled by valves 51, 52, 61, 62, 71 and 72. In the case of pure driver braking, the valves 51, 61 and 71 each assume their flow-through position, while the valves 52, 62 and 72 remain in a shut-off position. The valves normally used here are so-called 2/2-port directional control valves, i.e. valves with two ports and two switching positions (open and closed). In the case of pure driver braking, only the brake pressure FBD (driver brake pressure) generated in a known manner by the master cylinder 12 and optionally an additional brake booster 16 acts in the wheel brakes 50 and 60.
In the case of system braking, in contrast, the brake pressure SBD (system brake pressure) is generated by a pressure generator 18, as already explained above. For this purpose, the latter has an actuator 20 which drives a piston pump, in particular a multiple-piston pump 22. The latter can have, as shown in FIG. 1, for example three pressure chambers 241, 242, 243 (denoted generally by the reference symbol 24) for each brake circuit A and B, respectively, and associated pump pistons which act on the pressure chambers and are connected to the actuator 20 via a common eccentric. The multiple-piston pump 22 is connected, on the inlet side, to the storage container 14 of the brake system 10 and in the actuated state sucks in brake fluid therefrom which is delivered by the pump pistons in the direction of the wheel brakes 50 and 60 by the action of the pump pistons on the pressure chambers 241, 242, 243.
In the case of anti-lock control (ABS), locking of the wheels during braking is to be prevented. For this purpose, the brake pressure GBD (total brake pressure) acting on the wheel brakes 50 and 60 is set by a temporal sequence of pressure build-up, pressure maintaining and pressure reduction phases. This takes place by activating the valves 51, 52 and 61, 62 respectively associated with the wheel brakes 50 and 60. These valves are set in a known manner so that, after a desired brake pressure GBD has been reached, this is maintained during the pressure maintaining phase (closed valve position), while in a pressure build-up or pressure reduction phase the hydraulic fluid can flow to the wheel brake 50 and 60 or away from it (open valve position). For intermediate storage of the hydraulic fluid, use is made of a low-pressure storage reservoir 26, from which the hydraulic fluid is delivered back to the brake circuit A and B by the multiple-piston pump 22.
In the operating state of ABS control, there is usually a mixed operation of the brake system 10, since, besides the system-initiated pressure generation by the pressure generator 18, the driver additionally actuates the master cylinder 12 via a foot pedal 28.
In the case of normal driver braking, brake pressures in the range of up to about 80 bar usually occur, depending on the use of a brake booster 16 and/or the brake force introduced by the driver via the foot pedal 28. However, in the above-described mixed operation in connection with ABS control, pressures of about 200 bar can be generated in special driving situations. This can happen particularly if ABS control continues for quite a long time, for example because the roadway has a low coefficient of friction (μ low). In this case, the low-pressure storage reservoir 26 is completely filled, so that the pump pistons have to deliver against the prevailing brake pressure GBD and in so doing increase the latter further. At pressures of the order of about 200 bar, however, the actuator 20 driving the pump pistons may stop, as shown in practice, owing to the prevailing loading by the fluid pressure acting against the delivery direction of the pump pistons. This has the result that the entire ABS control fails and therefore cannot perform its safety-related function any longer.
In order to counteract such a failure, solution approaches from the prior art are known, in which the reaction load acting on the pump pistons is to be reduced by decreasing the active area of the pump pistons. However, decreasing the active areas of the pump pistons also has a detrimental effect on the performance of the multiple-piston pump 22 with respect to its delivery volume and the dynamics.